Method and apparatus for measuring the calorific value of a gas

ABSTRACT

A method and apparatus for measuring the calorific value of a gas. The apparatus includes a chamber to which a gas in question, for example natural gas, is supplied through an inlet and leaves through an outlet. The speed of sound SoS at ambient temperature is measured using any suitable method such as electronic control and a calculating device and an ultra-sound emitter and an ultra-sound receiver. The ambient temperatures T a , is observed by a temperature sensor, and a thermal conductivity sensor measures the thermal conductivity of the gas at two different temperatures above the ambient temperature. One value ThC H , of the thermal conductivity is measured at 70° C. above ambient and the other value ThC L  of the thermal conductivity is measured at 50° C. above ambient. The control calculates the calorific value CV of the gas according to the formula: 
     
       
         
           CV=a·ThC 
           H 
           +b·ThC 
           L 
           +C·SoS+d·T 
           a 
           +e·T 
           a 
           2  
           +f, 
         
       
     
     where a, b, c, d, e and f are constants.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for measuring thecalorific value of a gas.

The gas may be a fuel gas, for example natural gas. The natural gas maybe methane and may further comprise nitrogen and/or carbon dioxide. Inaddition to methane the natural gas may comprise at least one otherhydrocarbon gas, for example ethane, propane, butane, pentane or hexane.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method of measuring thecalorific value of a gas comprises making a measure of the speed ofsound in the gas and using the speed of sound in an operation producingthe calorific value corresponding to said speed of sound.

According to another aspect of the invention a method of measuring thecalorific value of a gas comprises making a measure of the speed ofsound in the gas, making a measure of a first thermal conductivity ofthe gas at a first temperature, making a measure of a second thermalconductivity of the gas at a second temperature which differs from thefirst temperature, and using the speed of sound and the first and secondthermal conductivities in an operation producing the calorific value ofthe gas corresponding to said speed of sound and said first and secondthermal conductivities.

According to a further aspect of the invention an apparatus to measurethe calorific value of a gas comprises means to measure the speed ofsound in the gas and means to use the speed of sound in an operationproducing the calorific value of the gas corresponding to said speed ofsound.

According to a still further aspect of the invention an apparatus tomeasure the calorific value of a gas comprises means to measure a firstthermal conductivity of the gas at a first temperature; means to measurea second thermal conductivity of the gas at a second temperature whichdiffers from the first temperature, and means using the speed of soundand the first and second thermal conductivities in an operationproducing the calorific value of the gas corresponding to said speed ofsound and said first and second thermal conductivities.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 diagrammatically shows an apparatus in which the invention can beperformed; and

FIG. 2 shows a diagrammatic example of a feed forward air/fuel gascontrol system utilising the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, an apparatus 2 to measure the calorific valueof a gas has a chamber 4 into which the gas is supplied through an inletconduit 6 and leaves through an outlet conduit 8. The inlet conduit 6includes heat exchange means 6A, for example, a copper coil, by whichthe temperature of the incoming gas can be adjusted to a valuesubstantially the same as that of the ambient temperature of theexternal atmosphere, whereby the gas in the chamber 4 is ofsubstantially uniform temperature throughout. The chamber 4 includes anultrasound emitter transducer 10 and an ultrasound receiver transducer12. An electronic control means 14 including computer means is connectedto a signal generator 16 so that under the control of the control means14 the signal generator causes the transducer 10 to emit ultrasoundsignals 18 as desired. The ultrasound signals 18 are received by thetransducer 12 and their reception signaled to the control means 14 vialine 20. The time of flight of the ultrasonic signals betweentransducers 10 and 12 is measured by the control means 14 which isarranged to calculate SOS which is the speed of sound in metres/second(m/s).

If desired some other means of measuring the speed of sound in the gasmay be used, such as that disclosed in U.S. Pat. No. 4,938,066. However,the most preferable method is that disclosed in UK patent applicationNos. GB 9813509.8, GB 9813513.0 and GB 9813514.8. These applicationsdisclose the use of a resonator to measure the speed of sound of a gaswithin the resonator. A driving electronic circuit which may include orbe in the form of a microprocessor is arranged to produce a sinusoidalsignal over a suitable range of frequencies to drive a loudspeaker. Theloudspeaker is arranged to apply an acoustic signal to the interior of aresonator. A microphone is arranged to detect the magnitude of theacoustic signal within the resonator. The signal from the microphone isfiltered and amplified by an appropriate electronic circuit and aprocessing means determines the resonant frequency relating to the gaswithin the resonator to determine its speed of sound.

A temperature sensor 22 in the chamber 4 provides the control-means 14with data on line 24 representing the value of the ambient temperature.

The ambient temperature sensor 22 may be part of a thermal conductivitysensor 28 comprising thermal conductivity observation means 30. Thethermal conductivity sensor 28 may be a miniature thermal conductivitymicrosensor model type TCS208 available from Hartmann & Braun AG ofFrankfurt am Main, Germany.

The thermal conductivity observation means 30 used to observe thethermal conductivity of the gas has heater means which in response tosignals on line 32 from the control means 14 can operate at more thanone selected desired temperature above the ambient temperature observedby the sensor 22, and a signal representative of the thermalconductivity of the gas at the desired temperature is sent to thecontrol means on line 34.

The control means 14 is arranged to cause the thermal conductivitysensor 28 to measure the thermal conductivity of the gas at twodifferent desired temperatures t_(H) and t_(L) in which t_(H), is apre-determined desired number of temperature degrees t₁, above theambient temperature observed by the sensor 22 and t_(L) is apredetermined desired number of temperature degrees t₂, above ambienttemperature; the number t₁, being greater than the number t₂.

Using the observed or measured values of the speed of sound in the gas,the thermal conductivity of the gas at temperature t_(H) and t_(L) andthe observed value of the ambient temperature of the gas by sensor 22,the control means 14 calculates the calorific value of the gas using theformula

CV=a·ThC _(H) +b·ThC _(L+) C·SoS+d·T _(a) +e·T _(a) ² +f  −I

in which

CV is the calorific value;

ThC_(H) is the thermal conductivity of the gas at temperature t_(H);

ThC_(L) is the thermal conductivity of the gas at temperature t_(L);

SoS is the speed of sound in the gas at the ambient temperature;

T_(a) is the ambient temperature of the gas observed by the sensor

22, and a, b, c, d, e and f are respective constants.

The gas in question may be a mixture of two or more gases in which thecomposition of the mixture may be of variable proportions. For examplethe gas in question may be a fuel gas. Such a fuel gas may be naturalgas. The natural gas may comprise methane and at least one of ethane,propane, butane, pentane or hexane, and may further comprise nitrogenand/or carbon dioxide.

In order to derive the constants a, b, c, d, e, and f in equation I, themathematical technique known as regression analysis may be used inrespect of data collected in connection with the gas in question. Theproportions of gases in the mixture may be varied to form a number ofdifferent samples. Using chromatographic methods, the calorific value CVof a sample is obtained, the ambient temperature T₃, of the sample ismeasured, and the thermal activities ThC_(H) and ThC_(L) of the sampleare measured. This is done for each sample in turn to obtain a set ofmeasured values corresponding to each sample. The sets of values areinserted in equation I and the “best-fit” values for constants a, b, c,d, e and f are derived. In the case of natural gas coming ashore at anumber of locations in the United Kingdom, regression analysis wasperformed on samples from the different locations and also on gasequivalence groups which are artificial replications in the laboratoryof mixtures of methane and ethane, methane and butane, methane andpentane, and methane and hexane in which, in the laboratory, thosemixtures are represented by different mixtures of methane and propane.

When equation I was applied to natural gas and to gas equivalence groupsand regression analysis used, the following values for the constantswere derived, namely:

a=36.25649,

b=−45.5768,

c=0.047029

d=0.091067,

e=0.00074, and

f=24.18731, when

CV is the calorific value of gas in MJ/m³ _(st)(Megajoules/standardcubic metres);

ThC_(H) is the thermal conductivity of the gas in W/m.K (where K isdegrees Kelvin) at a temperature of substantially 70 degrees Celsiusabove ambient temperature T_(a);

ThC_(L) is the thermal conductivity of the gas in W/m.K at a temperaturet_(L) which is substantially 50 degrees Celsius above ambienttemperature T_(a);

SoS is the speed of sound in the gas in m/s, and T_(a) is the ambienttemperature of the gas in degrees Celsius.

In the above application of equation I to natural gas, the value of t₁,is substantially 70° C. and the value of t₁ is substantially 50° C. Thusthe difference between the temperatures t_(H) and t_(L) at which thethermal conductivities ThC_(H) and ThC_(L) are measured differ bysubstantially 20° C. [(T_(a)+70)−(T_(a)+50)=20].

The value of the calorific value CV of the gas calculated by the controlmeans 14 may be visually displayed and/or printed or otherwise recordedby recording means 36 in response to signals from the control means.

By any suitable technique know per se the control means 16 may beprovided with information representing the relative density of the gasor the control means may be provided with information enabling it tocalculate the relative density RD of the gas. The control means 14 maycalculate or otherwise obtain the value of the Wobble Index WI of thegas using the formula. ${WI} = \frac{CV}{\sqrt{RD}}$

A method of measuring relative density is described in our co-pendingBritish patent application No. GB9715448.8 filed on Jul. 22, 1997.

When fuel gas is combusted in a process (e.g. furnace, kiln, compressor,engine, etc.) some form of control system is used to set the oxygen (inthis case in the form of air)/fuel gas ratio to ensure optimumcombustion. An allowance is made in the amount of excess air to accountin part, for variations in fuel gas composition changes. This allowancemeans that the process is running less efficiently than it could dobecause extra air is being heated and vented.

However, a measure of the calorific value or Wobbe Index, which isindicative of the fuel gas quality and which may be found according tothe present invention, may be used in a feed forward control strategy toimprove the accuracy of control available and achieve better efficiency.

An apparatus to perform such control is shown in FIG. 2. Fuel gas issupplied via a conduit 40, such as a pipe, to a gas fired process 41,such as a furnace, kiln, a compressor or an engine and oxygen in theform of air is supplied to the process 41 via another conduit 42. Anysuitable device 43 which may be in the form of one or more probestemporarily insert able into the conduit 40 or as one or more permanentfixtures is arranged to measure the speed of sound of the fuel gaspassing through the conduit 40, the thermal conductivities of the gasThC_(H), ThC_(L) at two temperatures t_(H) and t_(L), and the ambienttemperature of the gas T_(a), The speed of sound of the fuel gas SOS,the thermal conductivities ThC_(H), and ThC_(L), and the ambienttemperature of the gas T_(a) are measured by device 43 and passed via aconnection 44 to a control means 45, which may be a microprocessor or acomputer for example. Control means 45 determines the calorific value ofthe fuel gas from the received measurements from device 43 as explainedearlier. Having determined a measure of the gas quality, the controlmeans is able to adjust the air/fuel gas ratio setpoint using anoxygen/fuel gas ratio control system 46, 47 to achieve betterefficiency. In this case the oxygen/fuel gas control system comprisestwo variable opening valves 46, 47 one in each of the fuel gas and airconduits 40, 42 respectively and both controlled by the control means 45via connections 48, 49. Alternatively, the oxygen/fuel gas controlsystem could comprise a variable opening valve on just one of conduits40, 42.

What is claimed is:
 1. A method of measuring the calorific value of agas comprising: making a measure of a first thermal conductivity of thegas at a first temperature, making a measure of a second thermalconductivity of the gas at a second temperature which differs from thefirst temperature, and using a speed of sound and the first and secondthermal conductivities in an operation producing the calorific value ofthe gas corresponding to said speed of sound and said first and secondthermal conductivities.
 2. A method as claimed in claim 1, in which thecalorific value is obtained by a procedure involving use of the formula:CV=a·ThC _(H) +b·ThC _(L) +C·SoS+d·T _(a) +e·T _(a) ² +f, where CV isthe calorific value of the gas, where ThC_(H) is the first thermalconductivity of the gas at said first temperature, where ThC_(L) is thesecond thermal conductivity of the gas at said second temperature whichis lower than said first temperature, where SoS is the speed of sound ingas at ambient temperature, and where T_(a) is the ambient temperatureof said gas whereof said thermal conductivities are measured, the firstand second temperatures being greater than said ambient temperature, anda, b, c, d, e, and f are constants.
 3. A method as claimed in claim 2,in which SoS is the speed of sound in m/s, the thermal conductivitiesare in units of Watts/metre x degrees Kelvin (W/m.k), the temperatureT_(a) and the first and second temperatures are in degrees Celsius, andthe calorific value is in megajoules/standard cubic metre (MJ/m³ _(st)).4. A method as claimed in claim 2, in which the gas is fuel gas.
 5. Amethod as claimed in claim 4, in which the fuel gas is natural gas.
 6. Amethod as claimed in claim 3, in which the gas is natural gas comprisingat least one hydrocarbon gas which is methane, and said natural gasfurther comprises nitrogen and/or carbon dioxide.
 7. A method as claimedin claim 7, in which: a is substantially 36.25649, b is substantially−45.5768, c is substantially 0.047029, d is substantially 0.091067, e issubstantially 0.00074, and f is substantially 24.18731.
 8. A method asclaimed in claim 2, in which the first temperature is substantially 70°C. above ambient temperature.
 9. A method as claimed in claim 2, inwhich the second temperature is substantially 50° C. above the ambienttemperature.
 10. A method of measuring the Wobbe index of gas using theformula ${WI} = \frac{CV}{\sqrt{RD}}$

in which WI is the Wobbe index, RD is the relative density of the gas,and CV is the calorific value obtained by the method as claimed inclaim
 1. 11. An apparatus to measure the calorific value of a gascomprising: means to measure a first thermal conductivity of the gas ata first temperature; means to measure a second thermal conductivity ofthe gas at a second temperature which differs from the firsttemperature; and means using a speed of sound and the first and secondthermal conductivities in an operation producing the calorific value ofthe gas corresponding to said speed of sound and said first and secondthermal conductivities.
 12. An apparatus as claimed in claim 11, inwhich the calorific value is obtained by a procedure involving use ofthe formula: CV=a·ThC _(H) +b·ThC _(L) +C·SoS+d·T _(a) +e·T _(a) ² +f,where CV is the calorific value of the gas, where ThC_(H) is the firstthermal conductivity of the gas at said first temperature, where ThC_(L)is the second thermal conductivity of the gas at said second temperaturewhich is lower than said first temperature, where SoS is the speed ofsound in gas at ambient. temperature, and where T_(a) is the ambienttemperature of said gas whereof said thermal conductivities aremeasured, the first and second temperatures being greater than saidambient temperature, and a, b, c, d, e and f are constants.
 13. Anapparatus as claimed in claim 12, in which SoS is the speed of sound inm/s, the thermal conductivities are in units of Watts/metre x degreesKelvin (W/m.k), the temperature T₃ and the first and second temperaturesare in degrees Celsius, and the calorific value is inmegajoules/standard cubic metre (MJ/m³ _(st)).
 14. An apparatus asclaimed in claim 13, in which the gas is natural gas comprising at leastone hydrocarbon gas which is methane, and said natural gas furthercomprises nitrogen and/or carbon dioxide.
 15. An apparatus as claimed inclaim 14, in which: a is substantially 36.25649, b is substantially−45.5768, c is substantially 0.047029, d is substantially 0.091067, e issubstantially 0.00074, and f is substantially 24.18731.
 16. An apparatusas claimed in claim 12, in which the gas is fuel gas.
 17. An apparatusas claimed in claim 16, in which the fuel gas is natural gas.
 18. Anapparatus as claimed in claim 12, in which the first temperature issubstantially 70° C. above ambient temperature.
 19. An apparatus asclaimed in claim 12, in which the second temperature is substantially50° C. above the ambient temperature.
 20. A control means for adjustingthe oxygen/fuel gas ratio of a gas fired process comprising an apparatusfor measuring the calorific value of a fuel gas for the gas firedprocess according to claim 11, and means for adjusting an oxygen/fuelgas ratio control system for the gas fired process in accordance withthe measured calorific value.
 21. A furnace comprising means forreceiving a supply of oxygen; means for receiving a supply of fuel gas;an oxygen/fuel gas ratio control system; and a control means accordingto claim
 20. 22. A kiln comprising means for receiving a supply ofoxygen; means for receiving a supply of fuel gas; an oxygen/fuel gasratio control system; and a control means according to claim
 20. 23. Acompressor comprising means for receiving a supply of oxygen; means forreceiving a supply of fuel gas; an oxygen/fuel gas ratio control system;and a control means according to claim
 20. 24. An engine comprisingmeans for receiving a supply of oxygen; means for receiving a supply offuel gas; an oxygen/fuel gas ratio control system; and a control meansaccording to claim
 20. 25. An apparatus to measure the Wobbe index ofgas using the formula ${WI} = \frac{CV}{\sqrt{RD}}$

in which WI is the Wobbe index, RD is the relative density of the gas,and CV is the calorific value obtained using an apparatus as claimed inclaim 11.